This application claims priority from Canadian patent application number 2,342,119, filed Mar. 23, 2001, the disclosure of which is incorporated by reference herein in its entirety.
The present invention relates to testing process and apparatus for determining absorption properties of a porous specimen, and more particularly to testing process and apparatus for automatic measurement of water absorption and characterization of the absorption capacities and properties of porous materials, for example construction materials such as concrete, mortar, brick, stone, bituminous coated material, plaster, etc.
Deterioration of construction works (bridges, viaducts, tunnels, roads . . . ) and of buildings is mainly due to water which seeps by capillarity in the construction materials such as brick, stone, concrete. The mechanisms responsible for the deterioration of building materials are largely mediated by water and have been much described in specialized literature. The conjugate action of mechanical and chemical damages arising for instance from freeze/thaw cycles, shrinkage-induced cracking, salt crystallization and leeching (dissolution, selective migration of chemical elements, re-crystallization), interactions between the elements in solution or in suspension in water such as sodium and chloride ions or CO2 with the porous matrix or the framework, foam development, algae or fungi, may lead to rapid degradation of the bulk material properties of construction or building materials and significantly reduce the service life of the construction works. The durability of building structures is thus critically determined by the rate at which water and the many deleterious chemical agents it contains infiltrate and move through the porous structure. It is thus essential to quantify the absorption properties of the materials to estimate and eventually improve the durability of the construction works. This need, recognized very early, has motivated a very large number of studies since the beginning of the century and this effort is actually sustained.
One of the most currently used process for characterizing the absorption capacities of porous materials consists of measuring weight change in a specimen having an end brought in continuous contact with water. This process allows obtaining the quantity of water penetrating by capillarity in the material as a function of time. This process, generally called absorption test in the specialized literature, is classically used in construction engineering. It has been the object of many recommendations from different international organizations (see for example: xe2x80x9cStandard Test Methods for Sampling and Testing Brick and Structural Clay Tilexe2x80x9d ASTM, 2000, C67-99a, 1-10; xe2x80x9cConcrete Test Methodsxe2x80x9d, Tentative Recommendations, RILEM, 1972, Materials and structures, 7, 291-296) and has been described in numerous publications (see for example: xe2x80x9cWater sorptivity of mortars and concretes: a reviewxe2x80x9d, C. Hall (1989), Magazine of concrete Research, 41, 51-61; xe2x80x9cBarrier performance of concrete: A review of fluid transport theoryxe2x80x9d, C. Hall (1994), Material and structures, 1994, 27, 291-306). In all of these documents, it is suggested to carry out the weighing operation manually. The sample or specimen is drawn out of the reservoir at a regular time interval, wiped to eliminate excess water at the surface of the test specimen, weighed on a precision scale, and then set back in contact with water. This protocol is easy to practice but presents many flaws that may critically affect the quality of the measurements. The absorption process must be stopped during the weighing operation: it is thus not a continuous type of measurement. The consequences of these repeated interruptions on the measurement of the absorption properties are particularly important in the first minutes of the test. A residual amount of water at the surface of the sample when it is weighed may be largely greater than the quantity of water really absorbed by the sample at the beginning of the test. It is worth mentioning that the above-cited documents provide no indication on a way of estimating or reducing this error. The fact that many manipulations are required to measure the quantity of absorbed water considerably reduces the frequency at which the measurements can be carried out.
The results of the absorption test, most often obtained over periods of one hour or less, are then extrapolated for the typically much longer times of the service life of the construction works and buildings using unsaturated flow theory, a description of which is given in the above-mentioned documents of C. Hall. The theoretical model, routinely used since about 20 years in construction engineering, predicts that the increase of the quantity of water absorbed by capillarity during the absorption test proportionally varies as t1/2 (where t is the time during which the sample is in contact with water). The proportionality constant, called sorptivity, is generally interpreted as an intrinsic physical property of the material which characterizes its capacity to absorb water by capillarity. The fact that the content in water is initially null in the material is generally not considered (even though this condition is imposed) and the measurements carried out at the beginning of the test are neglected in the calculation of the sorptivity, as it appears from the above-mentioned documents of C. Hall and in xe2x80x9cA water sorptivity test for mortar and concretexe2x80x9d, B. B. Sabir et al., 1998, Materials and structures, 31, 568-574.
The unsaturated flow theory does not satisfactorily account for the infiltration process in porous milieu. The Applicant has discovered that the absorption capacities of porous materials are in fact largely higher than those predicted by the classical model. There is a need to improve the quality of the measurements of the water quantity absorbed by the material from the beginning of the absorption test. Likewise, there is a need to improve the absorption test to reach a correct estimate of the absorption capacities of construction materials.
The long time predictions of both the amount and the penetration depth of absorbed water based on the classical t1/2 relation are generally underestimated. Because this may also apply to many deleterious chemical agents mediated by water, the consequences of water infiltration on the durability of building materials may also be dramatically underestimated. The experimental procedure should be improved to increase the number and accuracy of cumulative infiltration data, especially at short times, to allow more reliable estimates of the absorption properties of porous building materials.
An object of the present invention is to provide testing process and apparatus which provide more reliable estimates of the absorption properties and behavior of porous materials in order to reach a better diagnostic on the durability of construction works.
Another object of the present invention is to provide testing process and apparatus which provide more accurate cumulative infiltration (water absorption) measurements even at the very beginning of the test.
Another object of the present invention is to provide testing process and apparatus allowing a continuous weighing of a porous specimen during water absorption.
Another object of the present invention is to propose testing process and apparatus for improving the estimation of the absorption properties of construction materials.
Another object of the present invention is to provide a testing process which can be practised in any office and laboratory and by any service involved in the design, construction or restoration of construction works and buildings.
Another object of the present invention is to provide such a testing process and apparatus which are applicable for unsaturated porous materials such as concrete, brick, stone, bituminous coated material, plaster, and the like.
According to the present invention, there is provided a testing process for determining absorption properties of a porous specimen, comprising: bringing the porous specimen in a predetermined state of contact with the body of water for inducing water absorption in the porous specimen; monitoring a weight of the porous specimen for a predetermined time period once the contact is established to produce weight data over time; maintaining the predetermined state of contact substantially constant during the predetermined time period; and determining the absorption properties of the porous specimen as a function of the weight data over time.
According to the present invention, there is also provided a testing apparatus for determining absorption properties of a porous specimen, comprising: a reservoir adapted to hold a body of water; a support extending over the reservoir, for hanging the porous specimen; a means for effecting a relative displacement of the reservoir and the support toward each other to bring the porous specimen in a predetermined state of contact with the body of water for inducing water absorption in the porous specimen; a means for maintaining the state of contact substantially constant during a predetermined time period; and a weighing means for measuring a weight of the porous specimen hanged by the support during the predetermined time period and providing weight data over time based on the weight; whereby the absorption properties are determinable as a function of the weight data.
Preferably, the testing apparatus also comprises a computer means connected to the weighing means, for determining the absorption properties as a function of the weight data over time.
The following provides a non-restrictive summary of certain features of the invention which are more fully described hereinafter.
With the testing apparatus according to the present invention, significant measurements of the quantity of water absorbed by the test specimen can be obtained as soon as the process begins. The frequency of the measurements during the test can be higher than in conventional process.
The analysis of the data resulting from the absorption measurements preferably uses an anomalous diffusion model developed by the Applicant to quantify the absorption properties of the porous materials subjected to testing with the present invention.
In short, the apparatus according to the present invention has a weighing device provided with a sensor to which the test specimen is hung. A water reservoir is provided for controlled immersion of a lowermost end of the specimen. An electronic device converts the analog signal produced by the sensor into a digital signal transmitted to a computer for data storage and analysis. A software component installed on the computer controls the test operations and analyzes the measurement data according to a predetermined model, preferably an anomalous diffusion model as developed by the Applicant. The weighing device, the water reservoir and the electronic device can be assembled on a rigid frame. The computer can be a PC type computer and may be provided separately from the testing apparatus.
The operation of the apparatus is based on the following general principles. The absorption of water by the specimen and the weighing are carried out simultaneously. The apparatus thus allows to record continuously the weight variations of the specimen during the absorption process. The weighing device is stationary, so as to avoid perturbations in the measurements which would be caused by a displacement of the sensor when the specimen is brought in contact with the water surface. The contact is preferably established by displacing vertically the reservoir which rests over a mobile platform. The water level is kept constant in the reservoir by permanently supplying the reservoir with water during the test. The water renewal in the reservoir allows, among other things, to keep the liquid at a constant temperature and thus to reduce experimental biases that could result from a change of physical properties.